Debt and Capital Lease Obligations (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	$ 400,000
Line of Credit Facility, Extension Capacity	50,000
Line of credit facility, maximum borrowing capacity	450,000
Performance bonds under credit facility	21,700
Line of credit facility, remaining borrowing capacity	293,300
Performance bonds	13,400
Long-term Debt, Excluding Current Maturities	233,739	$ 216,488
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt	150,000	150,000
Face amount	150,000
Line of Credit [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 85,000	68,000
Maturity date	Aug. 29, 2019
Senior Notes Series A [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.01%
Face amount	$ 75,000
Maturity date	Sep. 30, 2021
Senior Notes Series B [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.11%
Face amount	$ 75,000
Maturity date	Sep. 30, 2023
Total debt [Member]
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount	$ 235,000	218,000
Debt issuance costs [Domain]
Debt Instrument [Line Items]
Debt acquisition costs	$ (1,261)	$ (1,512)
Minimum [Member]
Debt Instrument [Line Items]
Basis Spread on Variable Rate	1.25%
Maximum [Member]
Debt Instrument [Line Items]
Basis Spread on Variable Rate	2.00%
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Derivative, Notional Amount	$ 50,000